I would like to log into my diskstation using ssh and public key auth. The keys are copied via ssh-copy-id and the files and permissions look good. I can also login successfully using the password method.
However my authorized_key file is not respected.

Unfortunately this article did not help. I had the user home service activated. When I login via ssh and password, I see that ~ is mapped to a folder specific to my user. I have added the .ssh folder and authorized keys as indicated.

Still public key login does not work. From the sshd traces it looks like the position of my user's home directory is not computed as expected (since it points to /root/.ssh instead of my user's directory). Unfortunately there is no logging the the ssh source code that traces how the folder name is computed.

However finally the trick was done by some privilege juggling.
The ssh-daemon denies to use keys stored in improper directories and files.
I was already astonished after running ssh-copy-id from my laptop that on DS218+ both, the directory has had 755 and the authorized_keys file 666 permissions. I set that accordingly. However first also let my own home-dir on DS218+ be set as 750 finally changed behaviour of sshd.